The present invention relates to a method for producing para-aramid pulp by means of gravity-induced shear forces and pulp made thereby.
The demand for para-aramid pulp such as the poly(p-phenylene terephthalamide) pulp sold under the trademark Kevlar.RTM. by E. I. du Pont de Nemours & Co. has been steadily increasing. Due to its high temperature stability, strength and wear resistance, para-aramid pulp is increasingly being used in brake linings and gaskets to replace asbestos. Para-aramid pulp is used in newly-developed papers, laminates and composites for applications requiring high strength and temperature stability; and para-aramid pulp is finding use as a reinforcing agent in composite elastomer structures such as in tires and hoses and the like.
U.S. Pat. No. 4,876,040 issued Oct. 24, 1989 on the application of Park et al., discloses a process for making pulp-like short fibers of aromatic polyamide by extruding a prepolymer dope into a coagulating liquid under shear conditions between the dope and the coagulating liquid.
U.S. Pat. No. 4,511,623 issued Apr. 16, 1985 on the application of Ycon et al., discloses a process for making pulp-like para-aramid fibers using a catalyzed, high-speed, high shear reaction with polymer of an inherent viscosity of greater than 5.0 dl/g.
Most para-aramid pulp is produced by spinning oriented, continuous fibers of the para-aramid polymer in accordance with a dry-jet wet spinning process such as that disclosed in U.S. Pat. No. 3,767,756 and then mechanically converting the fibers into pulp. The spinning of para-aramid fibers is an expensive and complicated process. U.S. patent application Ser. No. 07/358,811 filed June 5, 1989, now U.S. Pat. No. 5,028,372, in the name of Brierre et al. discloses a process for producing para-aramid pulp by means of extruding a polymerizing anisotropic solution of para-aramid, incubating the extruded solution to achieve a sufficient para-aramid molecular weight to gel the solution, cutting the gel, and isolating pulp from the gel. The extrusion in that process is necessary to achieve an orientation of para-aramid polymer molecules necessary for obtaining the pulp. Because the solution to be extruded is actively polymerizing, there is a tendency for the die to foul with stagnant polymer at the interface between the die and the solution.